Cable networks have been traditionally used to transmit video to users through their cable set-top boxes. The transmissions originate at a head-end, generally at a central location provided by the cable network provider. The transmission is received at the user end by a cable set-top box.
Using the infrastructure already established for the cable network, cable network providers began providing data services to users. With the use of a cable modem, users can transmit and send data across the network to other networks, such as packet-based networks like the Internet. The transmission travels upstream from the cable modem to the head-end, or a hub, where it is terminated by a cable modem termination system (CMTS). The CMTS then transmits the data out of the hub or head end to the packet network.
The CMTS also transmits the data from the packet network to the cable modem. Data is transmitted on one or more TV channels within the spectrum available for transmissions. The establishment, management and control of these channels is generally done using the Data-Over-Cable System Interface Specification (DOCSIS) standard. The standard allows the manufacturers of cable modems and CTMS equipment to ensure their equipment will function in the network, as well as allowing network providers to ensure that the network provides compatible services.
Currently, the DOCSIS specifications have a limited number of devices they can support per channel. A typical example is 8,191 modems per downstream channel, also referred to as a CMTS interface. This limit is due to the number of service identifiers (SIDs) used to identify a communication channel from the cable modem to the CMTS, referred to as an upstream channel.
Currently, SIDs are unique to a DOCSIS domain, where a DOCSIS domain may be defined as a collection of upstream and downstream channels for which a single MAC (media access control) allocation and management protocol operates. An upstream channel is a channel from the subscriber modem to the CMTS. A downstream channel is from a CMTS to a subscriber modem.
Two possible options to correct the issue of limitation of the number of supported devices per channel have been proposed. One proposal is to use more bits to define a SID, allowing a larger address space for the SIDs. However, this eliminates some of the SID space previously assigned as multicast/broadcast SIDs.
Another option would be to allow reuse of the SIDs within a DOCSIS domain, defining the SID space to be per upstream channel rather than per DOCSIS domain. This means that a SID on upstream channel 0 could be re-used on channel 1 within the same DOCSIS domain. Currently, if upstream channels 0 and 1 are in the same DOCSIS domain, a SID used on upstream channel 1 would not be used on channel 0. Using the same SID on different upstream channels eliminates some error capabilities that would otherwise be available.
For example, a CMTS receiving a packet on an upstream channel can currently check the validity of requests and messages by comparing the upstream channel identifier and the SID of received messages with the expected values. If the values do not match, the packet is discarded. With the above proposal, this type of checking is no longer possible because the combination of upstream channel ID and SID may be valid for all upstream channels.